1. Field of the Invention
This invention relates to a process and apparatus for manufacturing high purity product grade argon from an argon-containing stream using an argon column in combination with a solid electrolyte ionic/mixed conductor membrane.
2. Description of Prior Art
Conventionally, argon production by cryogenic air distillation is facilitated by an air separation unit employing a high pressure column and a low pressure column linked by a condenser/reboiler, with an argon side-arm stripping column (or "argon column") attached to the low pressure column. A fraction of vapor that rises through the low pressure column is diverted through a conduit therein to the base of the argon column. This vapor typically contains from about 5 to about 25% argon, and a small amount of nitrogen (typically, about 500 ppm), with oxygen and trace contaminants of the feed air stream constituting the balance thereof.
The function of the argon column is to further concentrate the argon content of the vapor--from the feed concentration of about 5 to about 25% to a level of about 98% or greater in the rising vapor. The argon column is refluxed by condensing most of the rising vapor using an argon condenser. Refrigeration for this condenser may be furnished from a number of sources including liquid from the base of the high pressure column, which itself is at least partially vaporized in the argon column condenser. The resulting liquid (or at least a major portion thereof) is introduced as reflux to the top of the argon column. As the reflux liquid descends within the argon column, the oxygen content of the reflux liquid increases. The descending reflux liquid is collected at the base of the argon column, and thereafter introduced to the low pressure column at or near the point where the argon column feed was originally withdrawn. An overhead product stream, either in vapor or liquid phase, is withdrawn from the argon column or the argon condenser, with the ratio of the argon column feed flow rate to product flow rate typically being about 25:1.
The relative volatility of argon to oxygen at the bottom of the argon column is about 1.5 and decreases to about 1.1 at the top of the column, where the product stream contains argon at a level of about 98% or greater. It is generally accepted that about 50 equilibrium stages in an argon column will produce argon containing less than about 2% oxygen, with an overall argon recovery of about 80% or greater based on the quantity of argon entering the air separation unit. Although some commercial argon applications can tolerate oxygen impurity levels as high as about 2%, most applications require the oxygen impurity level to be reduced to less than about 10 parts per million (ppm).
The elimination of oxygen to such an extent has heretofore necessitated further processing of the argon product withdrawn from the argon column. Such argon (sometimes referred to as "crude argon") has been further purified using catalytic deoxygenation, which first mixes an excess of hydrogen with the crude argon vapor and thereafter passes the mixture over a catalyst to form water. The water so formed is subsequently removed, such as by drying over an adsorbent.
While the quantity of oxygen removed by these conventional processes is relatively small, the cost and complexity of a catalytic deoxygenation process itself is significant. Such processes require heat exchangers, a catalytic reactor, an aftercooler, dual adsorbent dryer beds in cyclic operation and a cryogenic distillation column for excess hydrogen removal. In addition, a continuous supply of hydrogen is required, which makes these processes less attractive to geographic regions where hydrogen is either expensive or not readily available.
As an alternative to catalytic deoxygenation, oxygen impurities in crude argon may be reduced to acceptable levels by increasing the number of equilibrium stages within the argon column from about 50 to greater than 150. See, e.g., European Patent Publication EP 0 377 117. One common drawback with an argon column of 150 equilibrium stages or more is its physical height, which dominates the design and packaging of the air separation unit. Even with an argon column of 50 equilibrium stages, the combined height of the argon column and the argon condenser factors into determining the total height of the air separation unit when the liquid collected at the base of the argon column is to be returned to the upper column by gravity transfer. If a pump is used to return this liquid, an argon column of about 120 equilibrium stages can be used, provided the top of the argon condenser and the top of the low pressure column (when stacked above the high pressure column) are comparable in elevation. While this arrangement is advantageous for economically packaging the air separation unit, it is not capable of producing argon efficiently, particularly at the desired rate of recovery and level of purity. And further processing is then required to reduce oxygen impurities to an acceptable level.
Solid electrolyte membranes have been suggested to purify argon by removing-oxygen therefrom. See, e.g., U.S. Pat. Nos. 5,035,726 and its reissue Re. 34,595 (Chen). However, application of such membranes to purify argon requires compressors to elevate the pressure of warmed argon vapor, the use of which increases processing costs associated with impurity removal.
Thus, the development of a simple, cost effective method for recovering purified argon from an argon-containing stream would be highly desirable.
Accordingly, it is an object of this invention to provide an improved system for purifying and recovering product grade argon from an argon-containing stream.